1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount for damping or isolating vibrations based on flow of a fluid contained therein. More specifically, the present invention is concerned with such a fluid-filled elastic mount which utilizes a sub-atmospheric or vacuum pressure so as to exhibit different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto.
2. Discussion of the Related Art
As one type of vibration damping devices such as an engine mount for a motor vehicle, there is known a so-called fluid-filled elastic mount which includes a first and a second support structure that are spaced apart from each other and elastically connected to each other by an elastic body interposed therebetween. The fluid-filled elastic mount has a pressure-receiving chamber and a variable-volume equilibrium chamber which are filled with a suitable non-compressible fluid, and an orifice passage which premits flow of the fluid therethrough between the two fluid chambers. The pressure of the fluid in the pressure-receiving chamber changes upon application of vibrations. The fluid-filled elastic mount of the above type damps the input vibrations, based on resonance of a mass of the fluid flowing through the orifice passage, more effectively than an elastic mount which relies only upon the elasticity of the elastic body for damping the vibrations.
Generally, the elastic mount is required to exhibit different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto. For example, the elastic mount when used as a vehicle engine mount is required to exhibit high damping capability with respect to low-frequency vibrations, such as engine shake and bounce, and to provide a reduced dynamic spring constant with respect to middle- to high-frequency vibrations, such as engine idling vibrations.
However, the fluid-filled elastic mount constructed as described above can provide a sufficiently high damping effect based on the resonance of the fluid mass in the orifice passage, only with respect to the vibrations whose frequencies are in the neighborhood of the frequency to which the orifice passage is tuned. Therefore, it is extremely difficult for the known elastic mount to effectively damp or isolate two or more types of vibrations. Thus, the known elastic mount is not satisfactory in its vibration damping or isolating capability.
In view of the above, the assignee of the present application proposed a fluid-filled elastic mount having first and second support structures connected by an elastic body, a pressure-receiving chamber partially defined by the elastic body to receive applied vibrations, first and second equilibrium chambers partially defined by first and second flexible diaphragms, respectively, first and second orifice passages communicating with the pressure-receiving chamber and the first and second equilibrium chambers, respectively, as disclosed in U.S. patent application Ser. No. 07/718,425 filed Jun. 20, 1991 now U.S. Pat. No. 5,170,998. In the proposed elastic mount, the second orifice passage is tuned to a higher frequency than the first orifice passage. Further, a vacuum-receiving chamber is formed behind the second flexible diaphragm partially defining the second equilibrium chamber, such that the vacuum-receiving chamber is selectively exposed to the atmosphere or connected to a vacuum pressure source for supplying a sub-atmospheric pressure lower than the atmospheric pressure, so as to control elastic deformation of the second diaphragm and flow of the fluid through the second orifice passage.
In the thus constructed elastic mount, when the vacuum-receiving chamber is connected to the vacuum pressure source, the second flexible diaphragm is drawn onto the bottom wall of the chamber, whereby volumetric changes of the second equilibrium chamber and the fluid flow through the second orifice passage are prevented. Upon application of low-frequency vibrations, therefore, the fluid is forced to flow through the first orifice passage so that the mount provides a high vibration damping effect due to the first orifice passage. When the vacuum-receiving chamber is exposed to the atmosphere, on the other hand, the vacuum-receiving chamber having a given volume appears behind the second diaphragm, so as to allow elastic deformation of the second diaphragm and volumetric changes of the second equilibrium chamber Upon application of high-frequency vibrations, the fluid is forced to flow through the second orifice passage so that the mount provides a high vibration isolating effect due to the second orifice passage. Thus, the elastic mount exhibits different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto, based on the fluid flow through a selected one of the first and second orifice passages, by selectively connecting the vacuum-receiving chamber to the vacuum pressure source or the atmosphere.
Further study and analysis by the inventors of the present application revealed that the elastic mount as described above may not be able to stably provide a sufficiently high vibration isolating effect based on the fluid flow through the second orifice passage, because of the constructions of an air passage and a switch valve which are connected to the vacuum-receiving chamber, even when the vacuum-receiving chamber communicates with the atmosphere. Thus, the elastic mount as described above has a room for improvement in its vibration isolating capability.
Namely, even when the vacuum-receiving chamber of the above elastic mount communicates with the atmosphere, the air passage and switch valve apply resistance to the air flowing therethrough, whereby the vacuum-receiving chamber is not fully exposed to the atmosphere, and acts as if it were air-tightly enclosed. As a result, the vacuum-receiving chamber functions as an air spring, and is not able to sufficiently permit or accommodate the elastic deformation of the second diaphragm and the volumetric changes of the second equilibrium chamber. Consequently, effective flow of the fluid is less likely to occur through the second orifice passage, resulting in deterioration in the vibration isolating capability of the elastic mount.
To solve the problem as described above, it is proposed to employ an air passage and a switch valve having relatively large diameters, so as to reduce the resistance to the air flowing therethrough as much as possible. If the flow resistance in the air passage is excessively reduced, however, the elastic mount suffers from an increased rate or speed of evacuating the vacuum-receiving chamber when this chamber is connected to the vacuum pressure source. Upon connection of the vacuum-receiving chamber to the vacuum pressure source, therefore, a member which is supported by the first support member is likely to undergo displacement due to a shock or receive a vibrational load.
More specifically, when the vacuum-receiving chamber is connected to the vacuum pressure source, the volume of the second equilibrium chamber is increased as a result of elimination of the vacuum-receiving chamber which has been evacuated. As a result, the fluid in the mount is caused to flow from the pressure-receiving chamber to the second equilibrium chamber through the second orifice passage, in an amount corresponding to the increased volume of the second equilibrium chamber. At the same time, the fluid is caused to flow from the first equilibrium chamber to the pressure-receiving chamber, to compensate for a portion of the fluid which has been fed from the pressure-receiving chamber to the second equilibrium chamber. If the rate of evacuating the vacuum-receiving chamber is increased due to the reduced flow resistance in the air passage, the rate of reduction in the volume of the pressure-receiving chamber is so largely increased that the pressure of the fluid in the pressure-receiving chamber is undesirably lowered due to relatively slow fluid supply from the first equilibrium chamber through the first orifice passage. Due to the reduced pressure in the pressure-receiving chamber, the elastic body is deformed toward the pressure-receiving chamber, causing displacement of the first support member, whereby the member supported by the support member is likely to undergo shock-induced displacement or receive vibrations.